Known in the art are impact wrenches comprising a composite hammer including a driving part and a driven part between which there are disposed flyweights (cf. U.S. Pat. No. 3,804,180). Such impact wrenches exhibit high stability of blow energy and an adequate reliability. They may be used for tightening bolts made of medium-hard steel, as well as high tensile bolts.
When these impact wrenches are used for tightening superhigh tensile bolts, where high tightening torque is required, failures of heavily loaded components, in particular impact jaws occur. This is due to the fact that complete mating of impact jaws (in the axial direction) of the hammer and anvil occurs at a certain angle of engagement which is equal to the angular space between the respective jaws. To make the jaws stronger, their thickness is to be increased to result in a smaller angle of engagement. Moreover, the practice of operation of such impact wrenches shows that, with high tightening torques, the hammer and anvil should preferably be made with three jaws rather than two to make them stronger, which is impossible in the above-described impact wrenches.
Known in the art is an impact wrench (cf. U.S. Pat. No. 3,952,814) having a casing accommodating an axially immobile anvil having impact jaws, a composite hammer having impact jaws, and also a device for preliminary interaction of the hammer and anvil before engagement of their impact jaws, which enables the complete making of the impact jaws of the hammer and anvil in the axial direction during operation of the impact wrench in an automatic impacting cycle.
In the above-described impact wrenches, the driven part of the hammer is caused to move by flyweights (balls or rods) which requires a certain radial dimension to obtain a desired axial displacement of the driven part of the hammer so that such impact wrenches are only suitable for tightening large-diameter threaded fasteners.
Also known in the art is a pneumatic impact wrench comprising a casing accommodating a pneumatic motor having an output shaft, an anvil rigidly connected to a spindle, a composite hammer including a driving part coupled to the output shaft and a driven part received therein and connected thereto for a combined rotation and a relative axial movement. A guide roller is received in the driving part of the hammer, a return spring disposed between the driven part of the hammer and the anvil, and a stub shaft received in the anvil and connected thereto for a combined rotation and a relative axial movement. The stub shaft has a cam surface for co-operating with the guide roller (cf. Manual "Mekhanizirovanny Instrument, Otdelochnye Mashiny i Vibratory" (in Russian). TsNIITE Minstroydormash, 1975, pp. 179-180). Impact wrenches of this type featuring a positive displacement of the hammer into engagement with the anvil may be used for tightening threaded joints with high torque values. A substantially smaller angle of engagement is required in such impact wrenches to ensure complete mating of impact jaws of the hammer and anvil so that they may be provided with three jaws, respectively. Moreover, such impact wrenches feature a small radial dimension.
The impact wrenches of this type are, however, deficient in inadequate efficiency as each blow is delivered upon one revolution of the hammer. This does not permit a sufficient amount of energy to be accumulated and requires a high number of blows (about 120) to be delivered to tighten a threaded joint, which substantially reduces the efficiency of the process. Moreover, in view of the high blow rate, such impact wrenches exhibit an increased level of vibrations.